must or need


’ Need I tell them the truth?` Yes, you …

A: must
B: need

Could you please tell me why A is the correct answer and not B?


Hi Liza,

As it’s written, you can’t say ‘need’ again at the end of the sentence without following it with the preposition ‘to’. For example:

‘Need I tell them the truth? Yes, you need to.’

Since that isn’t a choice, you have to use ‘must’.

Why isn’t it a choice?

Note the above, the answer to your rhetorical question lies therein.

Because you need to.


Because you need “to”.


It’s always ultimately a “just because”, old chap. You could say that “must” takes a bare infinitive, while “need” takes a “to-infinitive”, and elaborate accordingly; but that would be simply to push the “just because” back a stage.


I’d say that “just because” could/should be fronted there, MrP:

Just because must" takes a bare infinitive, while “need” takes a “to-infinitive”, and so on…


Sorry to go back to this debate again, but could you please explain why ‘to’ is needed in the answer when in the question there is no ‘to’? In the question need is an auxiliary without ‘to’. So why isn’t it enough to repeat ‘need’ = the auxiliary in the short answer.
I don’t want to provocate, I just would like to understand the grammar point here (if there is).

thanks a lot

Hi Liza,

You’re certainly not provocative, you asked a legitimate question and got a unclear answer. Unfortunately, it’s hard to give a clear answer for this question.

‘Need to’ and ‘have to’ are treated as part of a group of special and rather irregular verbs called modal verbs. See here here for a list of ways modals are irregular.

Also, read our own thread for a discussion about the difference between ‘need’ and ‘need to’.

While some don’t consider them technically modal verbs, they’re often treated as such for convenience. ‘Need’ and ‘have’ require the use of the infinitive ‘to’ in order to differentiate them from the other, regular uses of the verbs ‘have’ and ‘need’. Thus, they join a list of other modals, such as:

Could do something → It’s possible for someone to do something.

Must do something → It’s absolutely necessary for someone to do something.

Need to do something → It’s important to do something.

Have to do something → It’s absolutely necessary for someone to do something and someone else requires it.

Mustn’t do something → It’s forbidden for someone to do something.

Couldn’t do something → Someone is unable to do something.

Don’t have to do something → It isn’t necessary for someone to do something.

Shouldn’t do something → It isn’t a good idea for someone to do something.

Should do something → It’s a good idea for someone to do something.

Might do something → It’s a possibility that someone might do something.

In summary, the best explanation I can give you for your question of ‘but why’, is twofold: 1) They’re modals (more or less) and thus irregular. Irregular verbs can’t be explained, that’s why they’re called irregular. 2) ‘Need’ and ‘have’ require the addition of ‘to’ when modalized to distinguish them from the regular form usage of those same verbs.

I realize 1) may be hard to accept just on the basis of ‘because it is’, but not everything can be logically explained in any language. For example, I can’t give you a solid explanation/reason for why we say ‘sing, sang, sung’, instead of ‘sing, singed, singed’ for the irregular verb ‘sing’, but we do.

English isn’t unique in having irregularities in its language, although it does seem to have more of them than many languages.

I hope this helps clarify your question somewhat.

As a perhaps redundant footnote:

“To need” without a to-infinitive can be used in negative or interrogative contexts, e.g.

  1. Need I tell them the truth? (interrogative)
  2. You needn’t tell them the truth if you don’t want to. (negative)

But in a positive context, a to-infinitive is required:

  1. You need to tell them the truth. (positive)

In your example, “Yes, you need…” is a positive statement; thus you can only say “Yes, you need to tell them” or simply “Yes, you need to” (where “tell them” is implicit).

(Sorry, I didn’t appreciate your point earlier.)

Best wishes,


Dear MrP,

Thanks for your answer. Now it’s clear. So in quesitons and negative sentences ‘to’ is not needed, but in positive ones it is necessary. I was looking for this kind of explanations. Thanks for it. Of course, now I won’t ask why. I can accept the rules of English grammar (when there’s a rule. And as it turned out here, there’s a rule here that I didn’t know before.). I would never ask why sing/sang/sung. I hate this kind of questions in my own language too. However, I think the ‘need to’ case is a little bit different.

However, I also think that ‘need to’ and ‘have to’ are a little bit different, because I need an auxiliary in the questions and negative sentences with ’ have to’, no?

Do I have to get up early? (I can’t say Have I to get I early? - Am I right?)
No, you don’t.
Yes, you do. / OR Yes, you have to. - By the way, are both two possible here??

Need I get up early?
No, you needn’t. (No, you don’t = isn’t it correct?)
Yes, you need to. (Yes, you do = isn’t it correct?)

Thanks a lot again.

Comments on the assertive versus non-assertive use of “need” anyone?

Indeed they are. The former is a full verb and the latter a semi-modal auxiliary.

Hello Liza,

In ordinary usage, that’s correct – e.g.

  1. Do I have to get up early?
    2a. No, you don’t (have to get up early tomorrow; but you will on Friday).
    2b. Yes, you do.
    2c. Yes, you have to.

  2. I don’t have to get up early, do I?
    3a. Yes, you do have to.

(2c and 3a would sound more emphatic: the speaker wants to stress the requirement.)

But this variant is not impossible, though it might sound old-fashioned or literary:

  1. Have I to get up early?

You might also hear (and this would be quite common, in BrE at least):

  1. Have I got to get up early?

Have a pleasant Tuesday,


In reply to that, I’ve heard “Yes/Of course you have.” in BrEng. Is it common?

It sounds quite distinguished to me.


Dear MrP,

Thank you so much for your previous answer, however, could you (or anybody else) please reply to my other question, too?

It was this:

Need I get up early?

  1. No, you needn’t.
  2. No, you don’t.
  3. Yes, you need to.
  4. Yes, you do.

Thanks a lot.

Hi Liza

We solve your “problem” quite nicely in AmE in that people would basically never say something such as “Need I get up early?” Instead, we’d generally say “Do I need to get up early?” Thus, the short answers would be either “Yes, you do” or “No, you don’t”.

Hello Liza,

I wouldn’t call any of those replies wrong, in BrE; “Yes, you do” suggests a “curt” reply, for instance, while “Yes, you need to” emphasises the “need”.

In ordinary usage, though, you might be more likely to encounter a reply such as:

  1. Not if you don’t want to.
  2. Yes, of course you need to get up early.
  3. You don’t need to get up early; but it wouldn’t hurt you if you did, for a change.

Best wishes,